How would I be able to set one vertices the same y axis as another one?
I assume &quot;setting a vertex to the same y axis as another vertex&quot; means that they both should be on the same line perpendicular to Y axis.
Snapping
You can use snapping for that. You can constrain to the other two axis by pressing shift+x,y or z so that transform operations are constrained to a plane and then hold ctrl while in transform operation to temporarily enable snapping. Your snapping settings need to be set to Vertex mode:

Scaling
You can also use scaling to 0 while using active element as transform pivot and constraining to the XZ axis plane the same way as with snapping. The active vertex will be the one you select the last.